1. Field of the Invention
The present invention relates to an air conditioner, and more particularly to an air conditioner employing a non-azeotropic mixture refrigerant which is formed of a high boiling-point refrigerant and a low boiling-point refrigerant.
2. Description of the Related Art
In general, a conventional refrigerant circuit of a heat-pump type air conditioner comprises a compressor, an indoor heat exchanger, an expansion (pressure-reducing) device, an outdoor heat exchanger and a four-way valve. In heating cycle, refrigerant in the circuit circulates therethrough in this order. In cooling cycle, the refrigerant circulates through the circuit in the opposite direction to the flow as described above.
In the case where a single refrigerant such as "R-22" (i.e., chlorodifluoromethane) is used in the conventional heat-pump type air conditioner as described above, such single refrigerant is constant in pressure, and its temperature is also constant in vapor/liquid mixed phase, so that there is no temperature glide therein.
In the air conditioner using the single refrigerant, when the refrigerant is controlled in the outdoor heat exchanger serving as an evaporator in heating cycle, frosting temperature detection is conducted at not only an inlet of the outdoor heat exchanger, but also at an intermediate portion of the outdoor heat exchanger because the temperature of the intermediate portion is reduced to a value lower than that of the inlet due to pressure loss of the refrigerant. In general, frosting occurs when the inlet temperature of the outdoor heat exchanger is equal to or lower than -3.degree. C.
Also, there has been recently known another conventional air conditioner which uses a non-azeotropic mixture refrigerant formed of a high boiling-point refrigerant and a low boiling-point refrigerant as a refrigerant for the air conditioner in view of prevention of depletion of a so-called ozone layer and other purposes.
When such a non-azeotropic mixture refrigerant is used in the air conditioner, the refrigerant is kept constant in pressure, and isothermal lines of the refrigerant obliquely extend from its saturated liquid line to its saturated vapor line in a down and right direction in vapor/liquid mixed phase as shown in FIG. 2 (i.e., obliquely extend toward the bottom right-hand corner of a graph shown in FIG. 2), so that a temperature glide occurs (for example, it corresponds to approximately 6.degree. C. at 5Kg/cm.sup.2). Consequently, in the air conditioner using the non-azeotropic mixture refrigerant, since the inlet of the outdoor heat exchanger serving as an evaporator becomes lowest in temperature due to the temperature glide of the refrigerant (provided that there is no other pressure loss of the refrigerant in the outdoor heat exchanger than the pressure loss due to the temperature glide), it is preferable to detect the temperature at the inlet of the outdoor heat exchanger when the frosting temperature detection is conducted.
The frosting of the outdoor heat exchanger serving as the evaporator depends on the outside air temperature. For example, even when the inlet temperature of the outdoor heat exchanger is equal to or higher than the frosting temperature, the frosting would often occur under such a condition that the outside air temperature is sufficiently low.
Consequently, when the expansion valve is controlled only according to the inlet temperature of the outdoor heat exchanger, it is not possible to prevent the frosting of the outdoor heat exchanger without fail.